S3.2.4 Boiling points

Factors affecting boiling point of organic compounds

Boiling point is influenced by:

Molar mass

Chain structure (straight vs branched)

Functional group (intermolecular forces)

Effect of molar mass on boiling point

As molar mass increases, boiling point also increases due to stronger intermolecular forces

Why higher molar mass increases boiling point

Larger molecules are more polarizable with more electrons and larger surface area, enhancing London dispersion forces

Boiling point trend in alkanes

Methane to decane show increasing boiling point with increasing molar mass due to stronger dispersion forces

Why branched isomers have lower boiling points

Branched isomers have lower boiling points than straight chain isomers with the same formula

Branches reduce surface contact area between molecules, weakening London dispersion forces

Effect of functional group on boiling point

Functional groups affect intermolecular forces

Stronger forces mean higher boiling points

Boiling point comparison: alcohols vs alkanes

Butan-1-ol (with hydrogen bonding) has a higher boiling point than pentane (only dispersion forces)

Functional groups with hydrogen bonding

Alcohols, amides, carboxylic acids can form hydrogen bonds, raising boiling point

Functional groups with dipole-dipole forces

Aldehydes, ketones, esters have dipole-dipole forces, weaker than hydrogen bonding

Functional groups with only London dispersion forces

Alkanes, alkenes, alkynes are nonpolar and rely only on weak London dispersion forces

Ranking organic compounds by boiling point

From lowest to highest boiling point:

alkanes/alkenes/alkynes < ketones/esters < alcohols/amides